Apparatus and methods for combined brightfield, darkfield, and photothermal inspection

ABSTRACT

Disclosed are methods and apparatus for detecting defects or reviewing defects in a semiconductor sample. The system has a brightfield (BF) module for directing a BF illumination beam onto a sample and detecting an output beam reflected from the sample in response to the BF illumination beam. The system has a modulated optical reflectance (MOR) module for directing a pump and probe beam to the sample and detecting a MOR output beam from the probe spot in response to the pump beam and the probe beam. The system includes a processor for analyzing the BF output beam from a plurality of BF spots to detect defects on a surface or near the surface of the sample and analyzing the MOR output beam from a plurality of probe spots to detect defects that are below the surface of the sample.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/618,586, filed 10 Feb. 2015, which claims priority to U.S.Provisional Patent Application No. 61/939,135, filed 12 Feb. 2014, whichapplications are incorporated herein by reference in their entirety forall purposes.

TECHNICAL FIELD OF THE INVENTION

The invention generally relates to the field of wafer and reticleinspection systems. More particularly the present invention relates toinspection and review of surface and sub-surface structures or defects.

BACKGROUND

Generally, the industry of semiconductor manufacturing involves highlycomplex techniques for fabricating integrating circuits usingsemiconductor materials which are layered and patterned onto asubstrate, such as silicon. Due to the large scale of circuitintegration and the decreasing size of semiconductor devices, thefabricated devices have become increasingly sensitive to defects. Thatis, defects which cause faults in the device are becoming increasinglysmaller. The device is fault free prior to shipment to the end users orcustomers.

There is a continuing need for improved semiconductor wafer inspectionapparatus and techniques.

SUMMARY

The following presents a simplified summary of the disclosure in orderto provide a basic understanding of certain embodiments of theinvention. This summary is not an extensive overview of the disclosureand it does not identify key/critical elements of the invention ordelineate the scope of the invention. Its sole purpose is to presentsome concepts disclosed herein in a simplified form as a prelude to themore detailed description that is presented later.

A system for detecting defects or reviewing defects in a semiconductorsample are disclosed. The system includes a brightfield (BF) module fordirecting a BF illumination beam on a BF spot on a sample and detectingan output beam reflected from the BF spot on the sample in response tothe BF illumination beam being directed on the BF spot and a modulatedoptical reflectance (MOR) module for directing a pump beam to a pumpspot on the sample, directing a probe beam to a probe spot on thesample, and detecting a MOR output beam from the probe spot in responseto the pump beam and the probe beam, wherein the probe spot iscoincident with the pump spot. The system also includes a processor thatis operable to perform or cause the following operations: (i) causingthe BF module to scan the BF illumination beam on a plurality of BFspots on the sample and detect an output beam from the plurality of BFspots, (ii) causing the MOR module to scan the pump and probe beam on aplurality of pump and probe spots, respectively, and to detect a MORoutput beam from the plurality of probe spots, (iii) analyzing the BFoutput beam from the plurality of BF spots to detect one or more defectson a surface or near the surface of the sample, and (iv) analyzing theMOR output beam from the plurality of probe spots to detect one or moredefects that are below the surface of the sample.

In a specific implementation, the BF module and the MOR module share anobjective. In another aspect, the system includes a darkfield (DF)module for directing a DF illumination beam on a DF spot on the sampleand detecting an output beam scattered from the DF spot on the sample inresponse to the DF illumination beam being directed on the DF spot. In afurther aspect, the processor is configured to cause the BF and DFillumination beam to scan the sample prior to scanning the pump andprobe beam and determine one or more target locations for scanning thepump and probe beam based on analyzing the BF and DF output beam afterthe entire sample or a portion of the sample is scanned by the BF and DFillumination beam. In yet another aspect, the BF and DF module share alight source for generating the BF and DF illumination beam. In anotherimplementation, the BF module has a BF light source for generating theBF illumination beam and the DF module has a DF light source forgenerating the DF illumination beam.

In a specific implementation, the MOR module comprises a pump lasersource for generating the pump beam at a wavelength range between about400 and 600 nm, a modulator for configuring the pump laser source tomodulate the pump beam, a probe continuous wave (CW) laser source forgenerating the probe beam at a wavelength range between about 600 and800 nm, illumination optics for directing the pump beam and probe beamtowards the sample, a photothermal detector, and collection optics fordirecting the MOR output beam towards the photothermal detector fordetecting the MOR output beam and generating an output signal that isfiltered to isolate changes that are synchronous with modulation of thepump beam. In a further aspect, the BF module comprises a BF lightsource for generating the BF illumination beam, illumination optics fordirecting the BF illumination beam towards the sample, a BF detector,and collection optics for directing the BF output beam towards the BFdetector for detecting the BF output beam. In a further aspect, theillumination optics of the BF module share one or more components withthe illumination optics of the MOR module and the collection optics ofthe BF module share one or more components with the collection optics ofthe MOR module.

In a specific embodiment, the defects that are below the surface includeone or more voids and/or changes in density of a material and/or changesin a sidewall angle and/or are within one or more through-silicon via(TSV) structure. In another aspect, the system includes an autofocusmodule for directing an autofocus beam towards the sample and detectinga reflected beam from the sample in response to the autofocus beam, andadjusting a focus of the system. In one aspect, the BF module, the MORmodule, and the autofocus module share an objective. In anotherembodiment, the processor is configured to cause the BF illuminationbeam, pump, and probe beam to scan simultaneously. In another aspect,the BF module and MOR module share a same detector for detecting the BFoutput beam and the MOR output beam.

In an alternative embodiment, the invention pertains to a method ofdetecting defects or reviewing defects in a semiconductor sample, andthe method includes (i) scanning a sample portion with a brightfield(BF) illumination beam, (ii) detecting a BF output beam reflected fromthe sample portion as the BF beam scans over the sample portion, (iii)determining surface or near-surface characteristics of the sampleportion based on the detected BF output beam, (iv) finding candidatelocations within the sample portion that are likely to have additionaldefects below a surface of the sample based on the determined surface ornear-surface characteristics of the sample portion that were determinedbased on the detected BF output beam, (v) directing a modulated pumpbeam and a probe beam at each candidate location, (vi) detecting amodulated optical reflectivity signal from each candidate location inresponse to each probe beam being directed to each candidate location,and (vii) determining a feature characteristic that is below the surfaceat each candidate location based on the modulated optical reflectivitysignal detected from such candidate location.

In a further aspect, the method includes (viii) detecting a darkfield(DF) output beam scattered from the sample portion as the BF beam scansover the sample portion or in response to scanning a darkfield beam overthe sample portion, (ix) determining surface or near-surfacecharacteristics of the sample portion based on the detected DF outputbeam, (x) finding a second plurality of candidate locations based on thesurface or near-surface characteristics of the sample portion based onthe detected DF output beam, (xi) detecting a modulated opticalreflectivity signal from each second candidate location in response toeach probe beam being directed to each second candidate location, and(xii) determining a feature characteristic that is below the surface ateach second candidate location based on the modulated opticalreflectivity signal detected from such second candidate location. In afurther aspect, the first and second candidate locations are found bycorrelating surface or near-surface characteristics with a presence ofsub-surface defects. In yet a further aspect, the first and secondcandidate locations are each associated with a sub-area of the sampleportion that has one or more surface or near-surface characteristicsthat deviate by a predefined amount from an average of the sampleportion. In yet another aspect, at least one of the first and secondcandidate locations has through-silicon vias (TSV's).

In another embodiment, the first and second candidate locations areselected to be distributed across its associated sub-area. In anotherexample, the method includes determining whether the surface ornear-surface characteristics indicate a presence of oxidation on thesurface and removing the oxidation prior to directing the modulated pumpbeam and the probe beam at each candidate location. In another example,at least some of the candidate locations are each selected to becentered on a structure based on an image of such structure that isgenerated based on the BF output beam. In another implementation, atleast some of the candidate locations are further selected to bedistributed across a structure based on an image of such structure thatis generated based on the BF output beam.

These and other aspects of the invention are described further belowwith reference to the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagrammatic side view of a Cu-filled through Silicon via(TSV).

FIG. 1B is a diagrammatic side view of a Cu-filled through Silicon via(TSV) having sub-surface defects.

FIG. 2 is a schematic representation of a combined brightfield (BF),darkfield (DF), and modulated optical reflectance (MOR) apparatus inaccordance with one embodiment of the present invention.

FIG. 3 is a diagrammatic representation of a combined BF and MORinspection system in accordance with an alternative embodiment of thepresent invention.

FIG. 4 is a flowchart illustrating an inspection procedure in accordancewith one embodiment of the present invention.

FIG. 5 is a flowchart illustrated a procedure for aligning MOR-basedillumination beams in accordance with a specific implementation of thepresent invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Thepresent invention may be practiced without some or all of these specificdetails. In other instances, well known component or process operationshave not been described in detail to not unnecessarily obscure thepresent invention. While the invention will be described in conjunctionwith the specific embodiments, it will be understood that it is notintended to limit the invention to the embodiments.

In general, certain embodiments of the present invention pertain to acombination system having channels for brightfield (BF), darkfield (DF),and modulated optical reflectance (MOR). A combined apparatus isespecially useful in detection and metrology of sub-surface defects,such as voids and other deformities in Cu-filled through Silicon via(TSV) structures or other 3D stacked semiconductor structures, as wellas detection and metrology of surface characteristics and defects, invarious structures on semiconductor samples, such as wafers.

FIG. 1A is a diagrammatic side view of a Cu-filled through Silicon via(TSV) structure 100. As shown, a Cu-filled TSV 108 is formed throughSilicon bulk 106, as well as active circuitry area 104 so as toelectrically couple a back metal portion 110 and one or more top metallayers 102.

BF- and DF-based technologies can detect surface and near-subsurfacedefects in various structures (e.g., in active circuitry area 104) invarious stages of the semiconductor manufacturing. However, certainfeatures in integrated circuit (IC) manufacturing such as Cu-filledTSVs, may be located much deeper in the semiconductor structure than thesensitivity depth of conventional optical-based macro inspectiontechniques. FIG. 1B is a diagrammatic side view of a Cu-filled throughSilicon via (TSV) having deep defects, such as void area 122 andparticle defect 120.

In contrast to BF- or DF-based technology, MOR-based technology can bevery sensitive to sub-surface features, such as defects located in TSVstructures and defects located in the bulk of the material. Therefore, acombination of these optical and photothermal technologies into a singleapparatus is expected to provide a tool capable of detecting defectsignatures throughout the entire range of interest (e.g., from <1 umto >50 um below the surface). Another advantage of such a combinedapproach lies in its simplicity. Unlike X-rays or ultrasonicwavelengths, a photo-thermal system is straightforward to build,maintain, and use.

FIG. 2 is a schematic representation of a combined apparatus 200 inaccordance with one embodiment of the present invention. The differentillumination and output beams of the system 200 are illustrated ashaving different optical paths so as to simplify the drawing and moreclearly show all the beams. The system 200 includes a photothermalMOR-based module and BF and DF based modules. The MOR, BF, and/or DFmodules may share any one or more components or be completely separatemodules. In the illustrated embodiment, the MOR and BF modules sharesome of the same illumination optics, such as objective 210. BF lightsource generates an illumination beam (black), which is directed towardsthe sample, e.g., wafer 216, via mirror 222 g, relay 204, mirror 222 d,and objective 210. Likewise, pump source 202 c generates a pump beam ofradiation (white), which is directed towards sample 216 via mirror 222 eand focused onto the sample by the same objective 210. Probe source 202d also generates a probe beam, which is directed towards sample 216 viamirror 222 c and focused onto the sample 216 by the same objective 210.Any of the illumination beams from any of the light sources describedherein may also pass through a number of lenses which serve to relay(e.g., shape, focus or adjust focus offset, filter/select wavelengths,filter/select polarization states, resize, magnify, reduce distortion,etc.) the beam towards the sample 216.

The sample 216 may also be placed on a stage (not labeled) of theinspection system 200, and the inspection system 200 may also include apositioning mechanism for moving the stage (and sample) relative to theincident beams. By way of examples, one or more motor mechanisms mayeach be formed from a screw drive and stepper motor, linear drive withfeedback position, or band actuator and stepper motor. The sample 216may be any suitable patterned or unpatterned substrate, such a patternedor unpatterned silicon wafer.

In general, each optical element of the inspection system or module maybe optimized for the wavelength range of the light for detecting defectsor characterizing features of the sample 216. Optimization may includeminimizing wavelength-dependent aberrations, for example, by selectionof glass type, arrangement, shapes, and coatings (e.g., anti-reflectivecoatings, highly reflective coatings) for minimizing aberrations for thecorresponding wavelength range. For example, the system optical elementsmay be arranged to minimize the effects caused by dispersion bywavelength ranges used by the BF and DF.

Any suitable BF source 202 a may be used. Examples of BF light sourcesinclude a coherent laser light source, laser-driven light source, (e.g.,deep UV or gas laser generator), a high-power plasma light source, atransillumination light source (e.g., halogen lamp), a filtered lamp,LED light source, etc. The inspection system may include any suitablenumber and type of light sources.

The BF source 202 a may generate any suitable broadband radiation at anysuitable incident angle, besides the illustrated normal angle. Forinstance, the BF illumination beam may be directed towards the sample216 at an oblique angle. The location at which the BF source is directedto on the sample may be referred to as a BF spot. The BF spot may bebetween about 0.5 μm to about 5 μm. In one example, the BF spot is about1 μm.

After the incident beam from light source 202 a impinge on the sample216, the light may then be reflected (and/or transmitted) and scatteredfrom the sample 216, which is referred to herein as “output light” or an“output beam.” The inspection system also includes any suitable lensarrangements for directing the output light towards one or moredetectors. The corresponding BF output beam is collected from the samplein response to such BF illumination beam. As shown, the normal outputbeam (black) is collected along the normal axis, through objective 210,mirrors 222 a, 222 b, 222 c, 222 d, 222 e, and reflected off opticalelement 222 h, split by beam splitter 222 j to impinge on both BFdetector 218 a and review camera 218 c. By way of example, the BFdetector 218 a may be in the form of a CCD (charge coupled device) orTDI (time delay integration) detector, photomultiplier tube (PMT), andother sensor.

The system 200 may also include a DF channel comprising a DF lightsource 202 b for generating a DF illumination beam (gray) towards sample216 at a particular angle, such as the illustrated oblique angle. Thelocation at which the DF source is directed to on the sample may bereferred to as a DF spot. The DF spot may be between about 0.5 μm toabout 10 μm, or more specifically between 0.5 μm and 5 μm. In oneexample, the DF spot is about 1 μm. The DF spot may be coincident withthe BF spot, and both the BF and DF spots may be generatedsimultaneously. Alternatively, the BF and DF beams may be simultaneouslydirected towards different spots on the sample. One example system thatincludes simultaneous BF and DF inspection is the Altair 8900 availablefrom KLA-Tencor Corp. of Milpitas, Calif.

The DF collection channel may be configured to collect scattered outputlight (gray) from the sample 216 in response to the incident DF beam. Asshown, scattered DF output light may be collected through objective 210,mirrors 222 a, 222 b, 222 c, 22 d, 222 e, and reflected off opticalelement 222 h and mirror 222 f to be directed DF detector 218 b.

The system 200 may also include a controller and analyzer 290 foranalyzing the output data from the BF and DF modules as describedfurther below.

The system 200 also includes a MOR-based module. The pump source 202 cgenerates an intensity-modulated pump beam of radiation. For example,the light source may include an intensity-modulated laser or incoherentlight source. A gas, solid state, or semiconductor laser may beutilized, including a laser system coupled with an optical fiber. Thepump source may generate a pump beam having any suitable wavelengthrange. For instance, the pump beam has a wavelength range that is lowenough to have strong absorption by the sample (e.g., silicon) whilekeeping within the bandwidth requirements of the shared illuminationoptics of the BF channel. In a specific implementation, the pump beamhas a stable wavelength range that is between about 400 to 600 nm. Abroadband tunable source may be used to achieve various wavelengths. Aspectroscopic pump source may be used to achieve better reflectivityfrom the sample.

The pump beam is intensity modulated. The pump beam may be configured tomodulate the pump beam at a predefined frequency or to vary themodulation frequency to obtain additional data at a plurality ofmodulation frequencies. Accordingly, the pump source will typicallyinclude a modulator 203 that supplies a drive signal to the pump sourcefor modulating the intensity of the pump beam. The modulation frequencycan vary from a few Hertz (Hz) to tens of MHz. In one implementation,the modulation frequency has a range between about 10 Hz and 10 MHz,such as 1 MHz, which will create plasma waves in a typical semiconductorsample.

As the pump beam source 202 c is switched on, the pump beam may beprojected onto the surface of the sample causing localized heating ofthe sample 216. As the pump source is modulated, the localized heating(excitation) and subsequent cooling (relaxation) may create a train ofthermal and plasma waves within the sample 216. The thermal and plasmawaves may reflect and scatter off various features and interact withvarious regions within the sample 216 such that the flow of heat and/orplasma from the pump beam spot may be altered. In an alternativeimplementation, the entire wafer may be relaxed in a special chamberand/or in a special environment utilizing temperature treatment.

Plasma waves are only generated in semiconductor materials and generallynot in conductive materials, such as copper, while thermal waves aregenerated in both semiconductor and conductive materials. Plasma andthermal waves propagate from the area of creation (e.g., the pump spoton the surface) to away from the surface. Plasma and thermal waves tendto be generated where the material's absorption coefficient is high,such as a silicon material, and reflect off any imperfections of thecrystalline structure or depend on particular structure characteristics.Imperfections or feature characteristics, which may affect plasma orthermal waves, may include voids, particles, missing or added material,changes in sidewall angle, a sidewall's deviation from vertical, densitychanges in material, etc. Thermal waves can penetrate quite deep, andthe depth of penetration can be changed by altering the intensitymodulation frequency of the pump laser.

The thermal and plasma waves and their interactions with the underlyingstructures or defects and their different absorption coefficients orthermal contrast will have a direct effect on the reflectivity at thesurface. That is, features and regions below the sample surface thatalter the passage of the thermal and plasma waves will alter the opticalreflective patterns at the sample surface. Thermal contrast generallydepends on differences in thermal conductivity multiplied by volume ordifferences in thermal diffusivity. If there is thermal contrast betweendefects and surrounding non-defective structures or field, significantchanges of surface reflectivity can occur.

By monitoring the changes in reflectivity of the sample at the surface,information about characteristics below the surface may be investigated.For detection of defects, the MOR-based module includes a mechanism formonitoring changes in reflectivity caused by the underlying structuresand/or defects. The system 200 includes a probe beam source 202 d thatgenerates an unmodulated probe beam of radiation. For instance, theprobe source 202 d may be a CW (continuous wave) laser or a broadband orwhite light source. The probe source may generate a probe beam havingany suitable wavelength range, depending on the reflectivitycharacteristics of the sample material. For instance, a probe beam thatworks well with copper has a wavelength range of about 600 to 800 nm. Inanother embodiment, the light source outputs illumination light at arange between about 700 nm and 950 nm (e.g., visible, IR, and/or NIRwavelength range) so as to penetrate a 3D stack memory device. Examplesof light sources include a laser-driven light source, a high-powerplasma light source, a transillumination light source (e.g., halogen orXe lamp), a filtered lamp, LED light sources, etc. Multiple LED orspeckle buster laser diodes are also possible sources.

The probe beam can be focused onto a probe spot that shares at least aportion of the same spot as the incident pump beam. That is, the probebeam impingement may be coincident with the pump spot. The system 200may also include a scanning element, such as a deflector, to move thepump and probe beams together so as to scan a same area of the sample.At least one beam tracker (e.g., 222 c or 222 a) may be provided in thepath of either the pump or probe beams for adjusting the relativepositions of the pump and probe beams on the sample. The tracker can beused to vary the lateral offset of the pump and probe beams forobtaining multiple MOR measurements. The probe beam may be directednormal to the sample surface (as shown) or at any suitable angle ofincidence. In another embodiment, the probe beam may be adjusted to ahigh power for flash annealing the sample 216.

The MOR channel may include a collection path for collecting outputradiation reflected or scattered from the sample 216 in response to thepump and probe beams incident on the sample. For instance the collectionpath may include any suitable lens or optics elements (e.g., 217) forrelaying and/or magnifying the output beam and directing it to aphotothermal (PT) detector 218 d.

The PT detector 218 d is generally configured to monitor the changes inreflectivity of the probe beam reflected from the sample in response tothe incident probe beam. The PT detector 218 d senses the outputradiation and generates an output signal that is proportional to thereflected power of the probe beam and is. therefore, indicative of thevarying optical reflectivity of the sample surface. The PT detector 218d may be in any suitable form to detect MOR-based signals, such asintegrated intensity signals. For example, the PT detector 218 d mayinclude one or more photodetector elements, such as a simple siliconphotodiode or an array of photodiodes. The PT detector 218 d preferablyhas low noise, high stability, and low cost.

The output signal from the PT detector 218 d may be filtered to isolatethe changes that are synchronous with the pump beam modulationfrequency. For many implementations, filtering may be performed using aheterodyne or lock-in detection system. U.S. Pat. No. 5,978,074describes several example lock-in detection systems, which patent isincorporated herein by reference in its entirety. A lock-in detector mayalso be used to measure both the in-phase (I) and quadrature (Q)components of the detector output. The two channels of the outputsignal, namely the amplitude A2=I2+Q2 and phase θ=arctan(I/Q) areconventionally referred to as the Modulated Optical Reflectance (MOR) orThermal Wave (TW) signal amplitude and phase, respectively.

Controller or analyzer 290 may also be configured for analyzing theoutput from the PT photodetector 218 d. In general, the phase and changeof the reflectivity signal, as compared to the modulated pump signal, ismonitored. Dynamics of the thermal- and carrier plasma-relatedcomponents of the total MOR signal in a semiconductor is given by thefollowing general equation:

$( {\frac{\partial R}{\partial T} + {\frac{\partial R}{\partial N}\Delta\; N_{0}}} )$

where ΔT₀ and ΔN₀ are the temperature and the carrier plasma density atthe surface of a semiconductor, R is the optical reflectance, δR/δT isthe temperature reflectance coefficient and δR/δN is the carrierreflectance coefficient. For silicon, δR/δT is positive in the visibleand near-UV part of the spectrum while δR/δN remains negative throughoutthe entire spectrum region of interest. The difference in sign resultsin destructive interference between the thermal and plasma waves anddecreases the total MOR signal at certain conditions. The magnitude ofthis effect depends on the nature of a semiconductor sample and on theparameters of the photothermal system, especially on the pump and probewavelengths.

The analyzer and processor 290 may also be communicatively coupled withone or more of the system components for controlling or sensingoperating parameters. For instance, the process 290 may be configured toadjust and control modulation of the pump beam via modulator 203.

The system 200 may also include an autofocus module 206 for providingautofocus of the target sample. The autofocus generally generates anautofocus beam that is directed by mirror 222 a through objective 210towards the sample and then detects a response signal to determine andadjust focus. In this embodiment, the autofocus shares the sameobjective as the BF and MOR-based modules.

FIG. 3 is a diagrammatic representation of an inspection system 300 inaccordance with an alternative embodiment of the present invention. Thissystem 300 may include one or more of the components described withrespect to the system of FIG. 2. As shown in FIG. 3, the system mayinclude a BF light source 302 a for generating an incident beam, such asa broadband light source. Examples of light sources include a coherentlaser light source, laser-driven light source, (e.g., deep UV or gaslaser generator), a high-power plasma light source, a transilluminationlight source (e.g., halogen lamp), a filtered lamp, LED light source,etc. The inspection system may include any suitable number and type oflight sources.

The incident beam from the BF light source 302 a then passes through anumber of lenses which serve to relay (e.g., shape, focus, resize,magnify, reduce distortion, etc.) the beam towards a sample 316. In theillustrated embodiment, the incident beam passes through lens 304, whichcollimates the incident beam, and then through lens 306, which convergesthe incident beam. The incident beam is then received by beam splitter312 that then reflects the incident beam through objective lens 314,which focuses the incident beam onto sample 316 at one or more incidentangles.

The inspection system 300 may also include an illumination selector 305positioned at a pupil plane of the illumination beam from light source302 a. In one embodiment, the illumination selector 305 is in the formof a configurable pupil aperture that is adjustable to produce aplurality of different illumination beam profiles at the pupil plane.The inspection system 300 may also include one or more positioningmechanisms for selectively moving the different aperture configurationsof the illumination selector into the path of the incident beam fromlight source 302 a.

After the incident beam(s) from light source 302 a impinge on the sample316, the light may then be reflected (and/or transmitted) and scatteredfrom the sample 316, which is referred to herein as “output light” or an“output beam.” The inspection system also includes any suitable lensarrangements for directing the output light towards one or moredetectors. In the illustrated embodiment, the output light passesthrough beam splitter 312, Fourier plane relay lens 320, imagingaperture 322, and zoom lens 324. The Fourier plane relay lens generallyrelays the Fourier plane of the sample to the imaging aperture 322. Theimaging aperture 322 may be configured to block portions of the outputbeams. For instance, the aperture 322 is configured to pass all of theoutput light within the objective numerical aperture in a bright fieldinspection mode, and configured to pass only the scattered light fromthe sample during a dark field inspection mode. A filter may also beplaced at the imaging aperture 322 to block higher orders of the outputbeams so as to filter periodic structures from the detected signal.

After going through the imaging aperture 322, the output beam may thenpass through any number of optical elements, such as beam splitters 332a, 332 b, and 332 c, and then through zoom lens 324, which serves tomagnify the image of the sample 316. The output beam then impinges upondetector 326 a. By way of example, the detector may be in the form of aCCD (charge coupled device) or TDI (time delay integration) detector,photomultiplier tube (PMT), and other sensor.

The system 300 may also include a pump source 302 b for generating apump beam that is directed by one or more lens and optical elements(e.g., 334 a, 332 a˜c, 320, 312, and 314) towards sample 316. Likewise,probe source 302 c generates a probe beam that is directed towards thesample by one or more lens and optical elements (e.g., 334 b, 332 b, 332c, 320, 312, and 314) towards sample 316. An output beam from sample 316is also directed towards PT detector 326 b by one or more lens (e.g.,314, 312, 320, 332, and 336).

The signals captured by the sensors of the above described systems canbe processed by a controller or analyzer system (290 or 310), which mayinclude a signal processing device having an analog-to-digital converterconfigured to convert analog signals from the sensor into digitalsignals for processing. The controller may be configured to analyzeintensity, phase, and/or other characteristics of the sensed light beam.The controller may be configured (e.g., with programming instructions)to provide a user interface (e.g., on a computer screen) for displayingresultant test images and other inspection characteristics as describedfurther herein. The controller may also include one or more inputdevices (e.g., a keyboard, mouse, joystick) for providing user input,such as changing aperture configuration, viewing detection results dataor images, setting up a inspection tool recipe. In certain embodiments,the controller is configured to carry out aperture selection orinspection techniques detailed below. Techniques of the presentinvention may be implemented in any suitable combination of hardwareand/or software. The controller typically has one or more processorscoupled to input/output ports, and one or more memories via appropriatebuses or other communication mechanisms.

The controller may be any suitable combination of software and hardwareand is generally configured to control various components of theinspection system. For instance, the controller may control selectiveactivation of the illumination sources, modulation of the pump source,illumination selector/aperture settings, the imaging aperture settings,etc. The controller may also be configured to receive the image orsignal generated by each detector and analyze the resulting image orsignal to determine whether defects are present on the sample,characterize defects present on the sample, or otherwise characterizethe sample. For example, the controller may include a processor, memory,and other computer peripherals that are programmed to implementinstructions of the method embodiments of the present invention.

Because such information and program instructions may be implemented ona specially configured computer system, such a system includes programinstructions/computer code for performing various operations describedherein that can be stored on a computer readable media. Examples ofmachine-readable media include, but are not limited to, magnetic mediasuch as hard disks, floppy disks, and magnetic tape; optical media suchas CD-ROM disks; magneto-optical media such as optical disks; andhardware devices that are specially configured to store and performprogram instructions, such as read-only memory devices (ROM) and randomaccess memory (RAM). Examples of program instructions include bothmachine code, such as produced by a compiler, and files containinghigher level code that may be executed by the computer using aninterpreter.

It should be noted that the above description and drawings are not to beconstrued as a limitation on the specific components of the system andthat the system may be embodied in many other forms. For example, it iscontemplated that the inspection or measurement tool may have anysuitable features from any number of known imaging or metrology toolsarranged for detecting defects and/or resolving the critical aspects offeatures of a reticle or wafer. By way of example, an inspection ormeasurement tool may be adapted for bright field imaging microscopy,darkfield imaging microscopy, full sky imaging microscopy, phasecontrast microscopy, polarization contrast microscopy, and coherenceprobe microscopy. It is also contemplated that single and multiple imagemethods may be used in order to capture images of the target. Thesemethods include, for example, single grab, double grab, single grabcoherence probe microscopy (CPM) and double grab CPM methods.Non-imaging optical methods, such as scatterometry, may also becontemplated as forming part of the inspection or metrology apparatus.

A brightfield (BF) and darkfield (DF) module can detect defects that areeither on the surface or covered by a very thin film. These modules cantypically detect undersurface defects only when they are closer to thesurface or they generate bumps in the stack that can propagate to thesurface and, therefore, create a detectable surface defect. TheMOR-based module can detect defects that are located deeper withinsample, such as at the bottom or on a sidewall of a TSV structure.

BF/DF and MOR-based modules can be used simultaneously to detect defectsor analyze characteristics on both the surface and deeper within thesample. Even for defects close to or on the surface, the BF/DF andMOR-based modules may be used together to enhance characterization ofsuch defect. The BF/DF and MOR-based inspections may result in differentdefects or characteristics for a same location. Analysis techniques forfinding defects and obtaining feature characteristics are furtherdescribed below. In general, the BF/DF and MOR-based modules may be usedto find defects that have different thermal and optical signatures.

In another embodiment, the BF/DF module and MOR-based modules can beused sequentially to enhance the inspection process. Although notrequired, the BF or DF modules may share a detector with the MOR-basedmodules since such modules are not used at the same time.

FIG. 4 is a flowchart illustrating an inspection procedure 400 inaccordance with one embodiment of the present invention. As shown, a BFis initially generated and directed towards a sample spot in operation402. Optionally, a DF illumination beam may also be directed towardssuch initial sample spot. A BF (and DF) output beam may then be detectedfrom the sample spot in response to the BF (and DF) illumination beam inoperation 404. BF and DF output data, such as detected signals orimages, may be simultaneously collected.

It may then be determined whether the last spot has been reached inoperation 406. For instance, it is determined whether the entire area tobe inspected has been scanned by the BF (and optionally DF) beam. Theentire sample or a portion of the sample may be scanned with the BF (andDF) beams. If the last sample spot has not yet been reached, the samplemay then be moved relative to the BF (and DF) illumination beam to scana next spot in operation 408.

The BF (and DF) beam may be scanned across individual swaths of thesample. For instance, the BF and DF beams are scanned across a firstswath in a first scan direction. These BF and DF beams may then bescanned across another a second swath in a scan direction that isopposite to the first swath's scan direction so that a serpentine scanpattern is implemented. Alternatively, the BF and DF beams may bescanned across the sample with any suitable scan pattern, such as acircular or spiral scan pattern. Of course, the sensors may have to bearranged differently (e.g., in a circular pattern) and/or the sample maybe moved differently (e.g., rotated) during scanning in order to scan acircular or spiral shape from the sample.

After the initial scan is completed, surface or near-surfacecharacteristics of the inspected area may then be determined based onthe detected BF (and DF) output beam to find candidate locations thatare likely to have additional defects below the surface in operation410. That is, candidate locations that may contain defects that are noton the surface or near the surface may be located by analysis of the BF(and DF) output data.

In one embodiment, die-to-die, cell-to-cell, or die-to-databasecomparisons are performed to locate differences that are above aparticular threshold, and these differences may then be classified asspecific defect types. Some of these defect types may be determined tocorrelate with deeper defects.

Different types of defects or characteristics can be seen by the BF andthe DF channels. BF output data can generally be used to detect defectsin the form of planar deformity of surface structures. For instance,some structures may appear larger or smaller than they are designed tobe. The BF output data can also indicate changes in film thickness, forexample, in the form of color changes. Color changes in the BF responsecan also be impacted by changes in the material itself (e.g., densitychanges). Additionally, groups of structures that have become defective(even if only individual structures within the group are defective) mayaffect the surrounding structures. In contrast, DF output data candetect particle defects and roughness on the surface, local gradients instructure height for a particular structure, etc.

Certain defect or sample characteristics types that are obtained from BFor DF output data may be used to find candidate locations for furtherMOR-based review. For instance, particular Cu-filled through Silicon via(TSV) or groups of TSV structures may have a difference in a particulardefect type (e.g., deformation) or characteristic (e.g., color), ascompared to other TSV structures. These TSV's may be selected ascandidate for further review by the MOR-based module.

In another example, the process is monitored using an averaging BF/DFtechnique to area anomalies. Averages in defect size, count, or othercharacteristics obtained from the BF/DF channels may be monitored perareas of the sample. For instance, a particular area may have biggerplanar structures than other areas with the same type of structures,such as TSV's. Areas that deviate by a predetermined threshold from theaverage or mean of the other sample area may be selected as candidatesfor deeper MOR-based review. Alternatively, a sampling of locationswithin a deviating area may be selected for further MOR-based review.For instance, a particular large area having a plurality of TSVstructures may be determined to have bigger structures, which may or notinclude TSV structures. A sampling of the TSV structures or all of theTSV structures within this particular large area may then be selectedfor MOR-based review. In one embodiment, candidate TSV locations areselected to be distributed across the particular area.

A pump and probe beam (e.g., from the MOR-based module) may be generatedand directed towards a first candidate location on the sample inoperation 412. A MOR-based output beam from the candidate location maythen be detected in response to the pump and probe illumination beam inoperation 414. A modulated pump beam is directed to a pump spot at thefirst candidate location on the sample, and a probe beam is directed toat least a portion of such pump spot. The reflected signal from theprobe beam is detected, for example, by a PT-detector as describedabove.

A feature characteristic below the surface at the candidate location maybe determined based on the MOR-based output beam in operation 415. Thereflectivity data obtained from the MOR-based module can be used to finddefects or further characterize features that are located deep in thesample at the candidate locations. Deeper defects and featurecharacteristics that can be monitored, measured, or found using aMOR-based review may include voids, particles, missing or addedmaterial, changes in sidewall angle, a sidewall's deviation fromvertical, density changes in a material, etc.

Training sets of known deep (or non-surface) characteristics and defectsmay be analyzed to determine their MOR-based output signal. Models mayalso be generated and trained to calculate particular characteristicvalues or defect types based on MOR-based reflectivity output data.

It may then be determined whether the last candidate has been reviewedin operation 416. If not, the sample can be moved relative to the pumpand probe illumination beam to scan a next candidate location inoperation 418. The MOR-based channel is used to collect MOR-based outputdata for each candidate location until the last candidate is reviewed.

After the last candidate location is reached, it may then be determinedwhether the sample passes in operation 420. For instance, it isdetermined whether the defects are yield-limiting defects or merelynuisance type defects. The BF, DF, and MOR-based defects and featurecharacteristics may all be analyzed to determine whether the samplepasses. It may also be determined whether particular featurecharacteristics are out of specification. If the sample does not pass,the process may be altered; the sample may be repaired, or the samplemay be discarded in operation 422. In one implementation, the sample isdiscarded and the process is altered. If the sample passes, theprocedure 400 may end, and the sample may be used as a product or befurther processed. After further processing, the sample may be againinspected.

Using the BF/DF channels in a first inspection pass to find candidatelocations for further MOR-based review can result in a more efficientinspection process. The BF/DF scans are significantly faster than aMOR-based scan. That is, the sample can be quickly scanned using a BFand/or DF inspection process. Suspect candidate locations can then bereviewed again by a MOR-based channel. The data that is obtained fromthe MOR-based review can be added to the data from the BF and/or DFinspection so as to provide a richer analysis for monitoring featurecharacteristics or finding defects on the sample.

The MOR-based output data can be used to improve the sensitivity of theBF and DF output data. In a TSV example, the thermal diffusion length inCopper can be varied significantly by adjusting the pump beam modulationfrequency. At the low modulation frequencies, the thermal diffusionlength and the sensitivity region of the system can be as large as50-100 um allowing characterization of deep voids in Cu TSV structures(40-60 um deep and 3-8 um wide).

The BF modules can also be used to determine whether the TSVs haveoxidized prior to inspection with a MOR-based module. For instance, asampling of TSV images may be reviewed to determine whether there isoxide present. The presence of oxide during a MOR-based probe of TSVstructures may cause the oxide to be burned off by the pump laser andcause unreliable results. It may be beneficial to remove the oxide fromsuch TSV structures or the like. If there is oxide present, the samplemay be polished (or some other technique implemented) so as to removeoxide. The MOR-based inspection can then proceed.

The BF module may also be used to align the pump and probe beams withrespect to a target structure. For instance, BF output data can be usedto align the pump and probe beam so as to accurately probe one or morespecific target locations of the target structure. In a specificexample, the target structure is a TSV structure. FIG. 5 is a flowchartillustrated a procedure 500 for aligning MOR-based illumination beams inaccordance with a specific implementation of the present invention.Initially, a BF illumination beam may be generated and scanned over atarget structure in operation 502. A BF output beam may then be detectedfrom the target structure in response to the BF illumination beam inoperation 504. In one example, an image of the target structure may begenerated based on the BF output beam.

The BF output beam may then be analyzed to determine one or more targetlocations on the scanned target structure in operation 506. In oneembodiment, the edges of the target may be determined, and the center ofthe target may then be determined for MOR-based probing. For example, aline of pixels may be obtained across the width (or radius) of thetarget structure, and such line can be used to find the center of thetarget structure. Alternatively, two orthogonal directions of pixels maybe obtained across the target structure to then locate a center oftarget structure.

If the target structure is large compared to the probe beam, a number oftarget locations may be selected to be distributed across the targetstructure so as to substantially cover the area of the target structure.In a distributed probing approach, a particular pattern of targetlocations may be chosen for systematically probing the target structure.In one example, a spiral pattern of locations that spiral out from thecenter of the structure is selected. In another example, a grid oflocations that can be scanned in a serpentine pattern across the targetis selected. In the TSV example, the TSV is probed so as to cover theentire TSV structure.

Once the one or more target locations are determined, a pump and probebeam may then be generated and directed towards a first target locationon the sample in operation 508. A MOR-based output beam is then detectedfrom the target location in response to the pump and probe illuminationbeam in operation 510. It may then be determined whether the last targetlocation has been reached in operation 512. That is, it is determinedwhether the probe pattern is complete. If the last target location hasnot been reached, the sample may then be moved relative to the pump andprobe illumination beam to scan the next target location in operation514.

After the last target location is probed, a feature characteristic ofthe target structure that is below the surface may then be determinedbased on the MOR-based output beam in operation 516, as furtherdescribed above. The alignment procedure 500 may be repeated on anynumber of target structures.

Although inspection systems and techniques for characterizing deepdefects or features are described herein as being applied to certaintypes of TSV structures, it is understood that embodiments of thepresent invention may be applied to any suitable 3D or verticalsemiconductor structures, such as NAND or NOR memory devices formedusing terabit cell array transistors (TCAT), vertical-stacked arraytransistors (VSAT), bit cost scalable technology (BiCST), piped shapedBiCS technology (P-BiCS), etc. The vertical direction is generally adirection that is perpendicular to the substrate surface. Additionally,inspection embodiments may be applied at any point in the fabricationflow that results in multiple layers being formed on a substrate, andsuch layers may include any number and type of materials.

The optical layout of the inspection/review tools can vary from thatdescribed above. For example, the objective lens can be one of manypossible layouts, as long as the transmission coatings are optimized forthe particular selected wavelength band or sub-band and the aberrationover each waveband is minimized. Any suitable lens arrangement may beused to direct each illumination beam towards the sample and direct theoutput beam emanating from the sample towards each detector. Theillumination and collection optical elements of the system may bereflective or transmissive. The output beam may be reflected orscattered from the sample or transmitted through the sample.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing the processes, systems, and apparatus of the presentinvention. Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein.

What is claimed is:
 1. A method of detecting defects or reviewingdefects in a semiconductor sample, the method comprising: scanning asample portion with a brightfield (BF) illumination beam; detecting a BFoutput beam reflected from the sample portion as the BF beam scans overthe sample portion; determining surface or near-surface characteristicsof the sample portion based on the detected BF output beam; findingcandidate locations within the sample portion that are likely to haveadditional defects below a surface of the sample based on the determinedsurface or near-surface characteristics of the sample portion that weredetermined based on the detected BF output beam; directing a modulatedpump beam and a probe beam at each candidate location; detecting amodulated optical reflectivity signal from each candidate location inresponse to each probe beam being directed to each candidate location;and determining a feature characteristic that is below the surface ateach candidate location based on the modulated optical reflectivitysignal detected from such candidate location.
 2. The method of claim 1,further comprising: detecting a darkfield (DF) output beam scatteredfrom the sample portion as the BF beam scans over the sample portion orin response to scanning a darkfield beam over the sample portion;determining surface or near-surface characteristics of the sampleportion based on the detected DF output beam; and finding a secondplurality of candidate locations based on the surface or near-surfacecharacteristics of the sample portion based on the detected DF outputbeam; detecting a modulated optical reflectivity signal from each secondcandidate location in response to each probe beam being directed to eachsecond candidate location; and determining a feature characteristic thatis below the surface at each second candidate location based on themodulated optical reflectivity signal detected from such secondcandidate location.
 3. The method of claim 2, wherein the first andsecond candidate locations are found by correlating surface ornear-surface characteristics with a presence of sub-surface defects. 4.The method of claim 3 wherein the first and second candidate locationsare each associated with a sub-area of the sample portion that has oneor more surface or near-surface characteristics that deviate by apredefined amount from an average of the sample portion.
 5. The methodof claim 4, wherein at least one of the first and second candidatelocations has through-silicon vias (TSV's).
 6. The method of claim 4,wherein the first and second candidate locations are selected to bedistributed across its associated sub-area.
 7. The method of claim 1,further comprising: determining whether the surface or near-surfacecharacteristics indicate a presence of oxidation on the surface; andremoving the oxidation prior to directing the modulated pump beam andthe probe beam at each candidate location.
 8. The method of claim 1,wherein at least some of the candidate locations are each selected to becentered on a structure based on an image of such structure that isgenerated based on the BF output beam.
 9. The method of claim 1, whereinat least some of the candidate locations are further selected to bedistributed across a structure based on an image of such structure thatis generated based on the BF output beam.